The original telephone system was developed to transfer sound over distance. Over the years, the capabilities on telephone lines have increased to the point where many functions can now be performed over the existing telephone lines. More features were added to the functionality of the telephone by the private address branch exchange (PABX).
The expanded functions of the telephone lines were enabled by the adoption of dual tone multifrequency (DTMF) signaling. Using DTMF signaling, digital information, such as dialing, could be passed from the telephone to the PABX as a type of audio signal. Telephone systems are limited in bandwidth to a range of between 200 Hz and 3,400 Hz. Because the time to transmit one bit of information is inversely proportional to the frequency, the rate of data transmission is affected by this bandwidth limitation. To increase the rate of data transmission, encoding has been used to allow more than one bit to be represented by one single pulse shape or signal level. This resulted a digital network for the telephone network.
The demands on use of such technology have expanded to the development of the integrated services digital network (ISDN) technology, an all-digital network. Basically, in an ISDN network all information, such as voice, data, and documents, still and moving images, is digitized and transmitted at high speed over a single, public switched network. All terminals linked to the network (e.g., telephones, computers, facsimile devices, video conference units, printers, or multimedia workstations combining several of these elements) use common connections. They use a single multi-purpose jack, common building wiring, and shared network facilities. The most common transmission method is to translate the binary digits into pulses, when are then transmitted across the telephone network. The information that is transmitted is digital in nature, and less susceptible to noise signals than corresponding analog signals.
A model for rationalizing the definition of complicated communications systems has been developed by the International Standards Organization. A method of fracturing a communications system into its constituent parts, known as the open systems interconnect (OSI) model, is the standard environment in which the ISDN operates.
Briefly, the OSI model splits the operation of a communications system into seven layers, each layer defining a different function within the communications system. The seven layers, arranged in order of increasing visibility to the user, are: physical layer, defining the physical interface responsible for sending and receiving the information across the network; data link layer, responsible for performing error detection and correction of data and for sending and receiving error-free data across the network; transport layer, responsible for controlling the flow of information over the connection; session layer, responsible for controlling the entire operation of connecting to the network; presentation layer, which defines how the information is presented both to the network for transmission and to the end-user; and, application layer, which defines the interaction between the user and the communications system.
Present available equipment utilizing the analog fax (GIII) and digital fax (GIV) technology require the use of multiple telephone lines: one dedicated to analog information and one dedicated to digital information. Often additional lines will be added to handle other signals, for example for transmission of electronic information used in electronic mail systems. The system rapidly becomes cumbersome and involves multiple systems, each designed to handle a single signal type, physically interconnected.
Facsimile machines traditionally have been designed to operate according to Group I, Group II, Group III, or some combination thereof through a modem apparatus into an analog telephone line that provides a link to similar equipment at a distant location. With the advent of Group IV facsimile, the compressed image is not presented to a modem but is in general passed directly through a digital telephone link, such as those now becoming commonplace under ISDN.
Since Group III is an established standard, recently introduced Group IV machines, such as the Canon L-3300 and the Ricoh 4500I, provide separate but comparable processing boards for Group IV and Group III. Specifically, Group IV is provided through a distinct processing board linked to a digital connection, such as an ISDN telephone connection, and Group III is provided through a distinct processing board linked to an analog connection, such as a POTS telephone connection. These connections typically have two separate telephone numbers and to the outside user may appear as two separate facsimile machines, as indeed they are. These two machines are placed in a single box and share a scanner, printer, and power supply, but otherwise function as distinct devices.
Thus, there remains a need for a system that utilizes ISDN capabilities to enable the transmission of a plurality of analog and digital signals across a single telephone line without the use of multiple independent systems.